The invention concerns a nondispersing infrared (IR) gas analysis device for the purpose of determining the concentration of a gas in a mixture of gases, where modulated IR radiation, after passing through the mixture of gases to be analyzed, arrives in a single-beam double-layer receiver with two receiver chambers filled with a selectively absorbing gas and arranged one after the other in the passage of the beam.
In devices of this type, the energy from the wavelength range which is most effectively absorbed by the receiver gas, for example, the energy from the center of an absorption line, makes up most of the energy absorbed in the front receiver chamber. As a result of the preliminary absorption in the front chamber, energy from less effective wavelengths, for example, the wavelengths located at the sides of an absorption line, makes up most of the energy absorbed in the rear, generally longer, receiver chamber. The difference in the radiation energies which are absorbed in the two receiver chambers, and the resulting difference in the thermal or pneumatic signals which are produced in the two chambers by this absorption, are indicative of the concentration of the gas which is to be measured in the mixture of gases. At the zero point of the measurement, that is, when the concentration of the gas which is to be measured is equal to zero, this difference in the signals is ideally equal to zero, meaning that the signals from the receiver chambers must have both the same amplitude as well as the same phase. These conditions cannot be fulfilled without difficulty in the case of a pure single beam arrangement in accordance with German Patent No. 1,017,385, corresponding to U.S. Pat. 2,951,939 of a source of radiation, an analysis cuvette, and the receiver, as a result of the different geometry and absorption distribution of the chambers. On the other hand, this device offers the advantage of simple construction and an optimal utilization of the radiation so that it is possible, by using it, to produce particularly small portable devices with a low consumption of electrical power.
For the purpose of solving the signal balance problem, it has already been suggested that the thermal time constants of the two chambers be adapted to each other by means of special designing of the shape of the rear receiver chamber, for example, see Russian Pat. No. 178,158. In accordance with another suggestion, the pneumatic balance is supposed to be achieved with the aid of supplementary capillaries and dead volumes, see German Patent (Auslegeschrift) No. 1,183,280 and similar U.S. Pat. No. 3,105,147. However, because of the unavoidable differences caused by manufacturing tolerances and the possible modification of the chambers from the outside over a period of time, these balance devices have to be adjustable without impairing the tightness of the vacuum, and thus these devices lead to considerable additional costs and increase the danger of the susceptibility of the devices to trouble.
In addition, in accordance with the German Patent (Auslegeschrift) No. 1,598,893 and similar U.S. Pat. No. 3,476,934, it was suggested that the signal transmitted by the receiver at the zero point of the measurement be compensated by means of an auxiliary signal with adjustable amplitude and phase. In doing this, the auxiliary signal is supposed to be produced with the aid of a second source of radiation modulated with the same frequency as the main source of radiation, or be supplied as an electrical signal to a different point in the measurement circuit formed by the receiver and amplifier.
The use of an auxiliary radiation source has the advantage that the compensation is accomplished at the start of the measurement circuit and that as a result of this, the auxiliary signal is subjected, to a large extent, to the same magnitudes of influence and disturbance as the main signal. However, this advantage must nevertheless be accepted along with higher costs for the mechanical-optical construction and for electrical power for the heating of the auxiliary source of radiation.